1. Field of the Invention
This disclosure generally relates to electrical power systems, and more particularly to power system architectures suitable for coupling generation resources to an electric grid via a common device.
2. Description of the Related Art
Traditionally, electric power is generated in large amounts at remote, large capacity generation stations. Typically, such large power stations are located close to an energy source and accordingly, are remote from the end users who consume the electric power. To minimize costs associated with the transmission of the generated electrical power to the end users, voltage of the generated electrical power is stepped up to a relatively high voltage for transmission over a high-voltage transmission system or grid. Voltage of the received electric power is then stepped down to a voltage that is useable by the end user, such as 240 volts (v) or 120 v.
However, it is increasingly difficult to build large power generation facilities and their associated high voltage, bulk power transmission line systems. For example, environmental pollution issues have made siting more difficult and very expensive, particularly when coal, gas, or hydro resources are located on remote, scenic public lands. Right of way for siting the high voltage, bulk power transmission line systems has become very difficult to procure.
An increasingly common trend in providing electric power to end users is the use of relatively small power generation units. Such smaller power sources may take advantage of readily available natural resources, such as solar energy, wind energy or geothermal energy. Smaller power sources may be referred to as distributed generator units when sited in close proximity to the end users of the electric power. Distributed generators may reduce or minimize transmission line facility costs.
Because of the wide variety of available resources used by smaller power sources, and because of the large number of manufacturers and vendors, there is relatively little uniformity in the design of the smaller power sources and/or their associated equipment. For example, some small power sources output alternating current (AC) power and other units output direct current (DC) power. Further, there is little uniformity in the output capacities, voltages and/or frequencies. For example, some small power sources may output power at a single phase voltage of 240 volts AC. Other small power sources may output three phase power at 480 volts AC. Some small power sources output AC power at a frequency of 60 cycles per second (Hz), typical of the standard frequency used in the United States, Mexico and Canada. Other types of small power sources output AC power at a frequency of 50 Hz, typical of the standard frequency used in Europe.
The above-described relatively small power sources may be electrically connected at a common location, such as at a substation that delivers power to the end users. When a relatively small power source is located near end users, it may be referred to as a distributed generator.
Electrically coupling a plurality of small power sources at a common location presents a variety of unique problems. Since there is little uniformity in the output characteristics of small power sources (i.e.; output voltage, frequency, or capacity), the interconnection equipment used to couple the small power sources to an AC power system will also vary widely. This interconnection equipment is very expensive. And, in many instances, the interconnection equipment is difficult and costly to install. For example, a plurality of co-located small power sources may have their grounding systems coupled together with external grounding connections or a common ground mat as a safety measure.
In some situations it may be very desirable to quickly electrically couple a plurality of power sources to a power system. For example, in the event of civil disorder, a natural disaster or marshal law event, the existing power supply may be disrupted. Repairing and restoring the damaged power system may take a very long time. Although some types of power sources are portable, it is problematic that interconnection facilities for connecting the mobile power sources to the power system will be readily available, and available in the most desirable locations.
Further, changes in operating conditions of one of the plurality of electrically connected small power sources will likely influence performance of other commonly located small power sources. For example, a voltage fluctuation on one small power source will, to some degree, cause a voltage fluctuation in the other small power sources. A fault or failure of a component in one small power source, or its associated equipment, may cause the voltage to collapse to zero volts or near zero volts until protective equipment isolates the faulted component. If the voltage fluctuation is sufficiently severe, some or all of the other small power sources may be forced off line. In a relatively small, electrically weak system, the voltage collapse will more readily propagate to the other small power sources and their associated equipment.
Accordingly, although there have been advances in the field, there remains a need in the art for electrically coupling a plurality of distributed generators at a common location. Some of the embodiments disclosed herein address these needs and provides further related advantages.